Electromotive power steering devices have a steering shaft that is configured by connecting an input shaft and an output shaft through a torsion bar. A steering wheel is connected to the input shaft side of the steering shaft, and a steering mechanism is connected to the output shaft side. Then, when rotating the steering wheel in one direction, steering torque applied to the input shaft is detected by the relative angular displacement in the rotational direction of the input shaft and output shaft produced by the intervention of the torsion bar, and the motor generates rotational force corresponding to the size of the steering torque in the direction of application of the steering torque. The device is configured such that this rotational force is transmitted to the output shaft, and output force augmented in the same direction as the direction of rotation of the steering wheel is transmitted from the output shaft to the steering mechanism.
The steering assist motor has a rotational shaft and an armature winding around the rotational shaft. Then, by supplying current to the armature winding, the rotational shaft rotates by electromagnetic action and rotational force is produced. In addition, the motor has a brush and a commutator that the brush contacts; and electric power is transmitted and supplied to the armature winding through the commutator by sending electric power from an outside source to the brush and the brush making contact with the commutator.
In this kind of motor, if the contact area between the brush and commutator is large, then a large volume of frictional noise is produced from the contact surface parts causing discomfort to the operator, and therefore a motor that reduces the noise caused by the frictional noise has been disclosed in Japanese Patent Application Publication No. 2006-311639 (“JP '639”). The motor described in JP '639 sets the contact area between the brush and the commutator based on the brush and the current flowing in the brush.